Plastic waste is now found in practically every part of the world, from the Antarctic coasts to the ocean depths. Faced with this dire fact of modern life, a team of marine scientists from the GEOMAR Helmholtz Centre for Ocean Research Kiel, the University of Kiel and the Cluster of Excellence “The Future Ocean” set out to find out how long it takes for plastics to decompose.

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They placed two common types of plastic waste – carrier bags and biodegradable bags – in oxic and anoxic sediments for 98 days. The carrier bags were conventional polyethylene bags while the biodegradable bags were made of compostable polyester, cornstarch and some undisclosed ingredients. The sediment samples taken from the Eckernförde Bay in the Western Baltic.

“In the upper layers of these sediment samples, oxygen was still present, but not in the lower layers. That is typical for seafloors around the world. […] These layers also differ in the types of bacteria that live therein,” said marine biologist Alice Nauendorf, author of the study.

After 98 days, the team examined the plastic bags for any changes in the material composition, using analytical methods such as high-precision weight measurements, scanning electron microscopy and Raman spectroscopy.

Publishing their results in the Marine Pollution Bulletin journal, the team declared that there was no change in the material of the bags. “We found no weight loss or chemical alteration. Therefore, no decomposition of the material is suggested,” said Professor Dr. Tina Treude, principal investigator of the study, who now works at the University of California, Los Angeles.

Nevertheless, the rate at which the biodegradable bags were colonised by bacteria was significantly faster. “We could clearly see that the compostable bags were more colonised with bacteria – in the oxygen-containing layers five times stronger, in the oxygen-free layers even eight times more powerful than the polyethylene bag,” said Nauendorf.

The reason for the difference is unknown, but Nauendorf suggested that an antibacterial substance in the polyethylene bags could have restricted the bacterial colonisation.

Nevertheless, it is obvious from the research that plastic degradation is a very slow process, and bacterial colonisation does not guarantee the chemical conversion of a substance. “The study suggests that the seafloor would become a long-term deposit for plastic waste if we don't stop polluting the seas. Future studies have to show what impact plastic waste has on benthic ecosystems,” said Professor Treude.